As is known in the art, flexible printed circuit boards typically are manufactured using substrates provided from woven materials such as pre-impregnated glass materials for example. This is especially true in the case of a so-called rigid-flex printed circuit board. Several of such substrates are typically bonded together by the inclusion of adhesive layers between the substrates. Such flexible printed circuit boards made from woven materials and conventional adhesives typically lack a homogeneous dielectric constant between adjacent inner layers.
As is also known, conventional manufacturing techniques provide printed circuit boards having an uneven surface topography. That is, the topography of a surface of the printed circuit board may vary, for example, between 0.010 inches and 0.020 inches. Such an uneven surface topography makes it relatively difficult to stack and drill several of such boards since the boards cannot be stacked flat during a drilling process. If the boards are not flat when stacked during drilling, then excessive burring occurs thus ruining the printed circuit boards.
Prior art approaches have typically used embossing materials to force the adhesive material to fill and flow around conductors on the substrate. Thus, the surface of the substrate had an uneven topography which did not allow the substrates to be stacked flatly. When substrates are not placed in a flat stack, it is relatively difficult to drill through the substrate since this would result in fracturing of the substrate through the back layer due to the uneven surface topography of the substrate. That is, the uneven surface topography results in the lack of proper support for each layer. This results in excessive burring of the printed circuit board when a drilling operation is performed on a stack of substrates. Thus, it is relatively difficult to drill through more than one substrate at a time. Therefore, prior art approaches typically drill holes through only one substrate at a time since the substrates are provided having uneven surface topographies.
Thus, flexible printed circuit boards are relatively expensive to manufacture because drilling and routing operations, for example, must be performed on one substrate at a time.
It would be desirable therefore to provide a printed circuit board having flat surfaces since this facilitates stacking and drilling of multiple printed circuit boards.